Generally, in order for fuel cells to obtain efficient power generation, system efficiency and long-period endurance of constituting members, an electrolyte membrane having good proton conductivity for an extensive time period at an operating temperature of 100° C. to 300° C. and under conditions having a relative humidity in the range of about 0% to about 50% may be required. An electrolyte member having these features has been considered in the development of conventional solid polymer electrolyte fuel cells. However, in a perfluorocarbonsulfonic acid membrane, sufficient proton conductivity and output power cannot be obtained at an operating temperature of 100° C. to 300° C. and at a relative humidity of less than about 50%.
A solid electrolyte membrane composed of polybenzimidazole doped with a strong acid, such as phosphoric acid has been disclosed in Japanese Patent Publication No. Hei 11-503262. While this type of solid electrolyte membrane has good oxidation resistance and thermal resistance and is operable even at a high temperature of 200° C., the membrane cannot obtain sufficient proton conductivity at a low relative humidity.